The present invention relates to a display device, and in particular to an active matrix type display device having a driver circuit for driving pixels, fabricated on its substrate of its display panel.
A display panel has an electrooptical material layer sandwiched between a pair of substrates. In this specification, the term “electrooptical material” refers to material which changes its optical properties such as transmission, emission, refractive index and absorption of light under the influence of an electric field or current. The electrooptical material includes liquid crystal material and electroluminescent material, for example.
By way of example, an active matrix type liquid crystal display device shall be considered.
In the active matrix type liquid crystal display device, each pixel area is surrounded by two adjacent ones of a plurality of gate signal lines extending in the x direction and arranged in the y direction and two adjacent ones of a plurality of drain signal lines extending in the y direction and arranged in the x direction which are fabricated on a liquid-crystal-layer-side surface of one of a pair of substrates sandwiching a liquid crystal layer. Each pixel area is provided with a thin film transistor operated by a scanning signal supplied from one of the gate signal lines and a pixel electrode supplied with a video signal from one of the drain signal lines via the thin film transistor.
The pixel electrode generates an electric field between it and a counter electrode fabricated on the other of the pair of substrates, for example, so that the electric field control the light transmission through the liquid crystal layer between the two electrodes. The liquid crystal display device is provided with a scanning signal driving circuit for supplying a scanning signal to each of the gate signal lines and a video signal line driving circuit for supplying a video signal to each of the drain signal lines.
These scanning signal driving circuit and video signal line driving circuit are formed of a large number of MIS (Metal Insulator Semiconductor) transistors having configurations similar to thin film transistors fabricated in the pixel areas, and therefore a technique is known in which semiconductor layers of the thin film transistors in the pixel areas are made of polycrystalline silicon (p-Si), and the scanning signal driving circuit and the video signal line driving circuit are fabricated on the one of the pair of substrates simultaneously with the pixels. These circuits are composed of transistors made of polycrystalline silicon, therefore their output signal levels are low, and consequently, their output signal themselves are sometimes insufficient for driving the pixels. To solve this problem, voltage level converters are incorporated into the liquid crystal display devices for converting voltages such as pulses from a low level to a higher level. Generally, the voltage level converters as shown in FIG. 16 or FIG. 17, for example.